Washington — There's a sign at Democratic Party headquarters in a room where a lot of volunteers worked on the presidential race. The sign says simply: ''Don't quit.''
Democratic loyalists are trying to keep that motto in mind after one of the most overwhelming election defeats in United States presidential history. But it's easy for pessimism to take over if anyone looks at the record:
* Democrats now have lost four of the last five races for the White House. Three of those elections were lost by landslide margins.
* Six times since World War II, Democrats have nominated presidential candidates from the North, such as Walter F. Mondale. None of them got 50 percent of the vote, although one, John F. Kennedy, eked into office with a tiny plurality.
* Two major groups of voters, both at the core of Democratic strength, have abandoned the party in presidential races. Both groups, Southern whites and Northern ethnics, could be difficult to win back.
Charles Manatt, the Democratic Party chairman, looked over the wreckage of Democratic hopes for the White House on Thursday and looked for the cause.
The party has failed to reach middle-class America, Mr. Manatt says. The typical voter that the party has lost might be described as a white man employed in a textile mill in North Carolina. The man has a wife at home. He is patriotic. His needs are simple. On weekends, he usually goes hunting or fishing.
Such voters, who supported Democrats almost by rote in the past, deserted Mr. Mondale and the Democrats by the millions on Tuesday. The reason: They're feeling pretty good about the country. They like President Reagan's strong foreign policy. They are employed. And inflation is in check, so their paychecks aren't shrinking.
Further, they are a bit puzzled by the Democrats. The party dwells on peripheral issues like gun control. It seems pointed in the wrong direction, talking about the need for higher taxes instead of more jobs.
Within hours after the voting on Tuesday, various experts were trying to figure out where Democrats should go from here. That debate will probably go on for most of the next four years.
Vic Fingerhut, president of Fingerhut/Grandos Opinion Research, was one of those sharply critical of Mondale's strategy this year, even though Mr. Fingerhut served on a Mondale advisory committee.
Fingerhut, like many other Democrats, was especially concerned about Mondale's emphasis on higher taxes. On Thursday, Fingerhut released a new study that he says indicates that Mondale would have done far better if he had hammered home on ''populist'' themes like jobs.
Fingerhut points out that it was the Democrats who deserted their party to vote for the President who handed Mr. Reagan his victory. Those Democrats could have been won back with the right appeal.
A poll of 800 voters on election day, says Fingerhut, showed that 75 percent of the Democrats who supported Reagan agreed that ''government should do more to protect ordinary Americans from the power of banks and big corporations.'' Some 83 percent want government to do more to create jobs.
Yet the same poll found that of these Democrats, 73 percent said that ''Reagan, rather than Mondale, seemed to be ... in the Democratic tradition of Roosevelt, Truman, and Kennedy in fighting for the working people.''
William Schneider, a resident fellow at the American Enterprise Institute, also suggests that one of the Democrats' biggest problems involves economics.
Democrats lost many conservatives in the 1968 and '72 elections, Mr. Schneider says. In 1980 and '84 their problems worsened, because the party lost ''economic credibility'' with the American middle class.
Where does the party go from here? Various analysts, including Schneider, see two possible scenarios.
First, the economy could suddenly turn downward. Then liberal Democrats - what Schneider calls the ''I told you so Democrats'' such as Sen. Edward M. Kennedy of Massachusetts - could quickly gain strength and the Republicans could be thrown out.
Second, Gary Hart-type Democrats could gain strength within the party, if the economic recovery under Reagan continues. Hart-type Democrats are seeking ways to absorb some successful Republican ideas, while maintaining some key Democratic traditions. They tend to be somewhat opposed to traditional big-government ideas of the Democratic Party, and are more inclined toward jobs creation in private industry.
There is wide agreement, however, that the party can hardly be expected to recapture the White House soon if it holds onto only to its current core, which is largely composed of upper middle-class liberals, blacks, and a few other groups, such as Mexican-Americans.
Norman Ornstein, another analyst at the American Enterprise Institute, says the picture really isn't so grim for Democrats when viewed in perspective.
After a thumping victory by President Eisenhower in 1956, Democrats roared back in 1958 races for Congress and got the White House in 1960. After Richard Nixon's huge win in 1972, Democrats quickly recovered (thanks to Watergate) and again won the White House in 1976.
A replay of those years could be at hand, says Dr. Ornstein. The key will be the congressional races in 1986.